Print marks are deployed as part of a register control process. The colors of the individual printing mechanisms are to be printed in the correct position in relation to one another. The print mark represents additional information, which is printed onto the material and serves only for control purposes. As well as being deployed for color register control, the print mark also serves for correct positioning during cutting (cut register control).
Naturally there also has to be a system, which detects and evaluates the printed print marks. Cameras are generally used for this purpose in modern register control systems. If it can be ensured that the camera photographs the print mark in a predefined region of its detection field, the detection time, i.e. the time when the camera is activated, can be related in a defined manner to the position of the print mark, thereby allowing control of the subsequent steps of printing further colors or even cutting.
For detection at the correct time, it is therefore initially essential for the print mark generally to be in the image range of the camera. Generally one print mark is located in the region of 1 m of print web. In contrast an image range of the camera is 60 mm in height. It is therefore imperative that the camera detects the print mark. The additional requirement that the camera should detect the print mark as centrally as possible is of no less importance.
In the prior art the camera is activated by way of a trigger system. Either the drive system, which controls the movement of the print web (i.e. the movement of the motors of the print cylinders transporting the print web), emits trigger signals directly and these are converted by the cameras or the drive system emits pulse generator signals, from which a trigger signal is derived in a separate unit.
The units generating the trigger signals are hereby generally relatively far away from the cameras for technical reasons. The signals therefore have to be routed over long distances at the machine. As these are time-critical signals, interference signal injection can cause incorrect activation of the camera system. Such interference originates particularly frequently from pulsed converters. Since the cables are laid by operators who frequently do not have adequate training, interference signal injection results to a significant degree in real systems.
EP 1 619 026 A2 discloses triggering an image recording unit by means of a control unit, namely as a function of a lead axle position of a lead axle defined in a printing machine. To allow this, the control unit communicates with at least one drive control system arranged downstream thereof of at least one group of pressure marks of the printing machine or also with a special processing and data processing unit.